Polyurethane construction adhesives offer an easy-to-use alternative to fasteners, such as nails or screws. One such polyurethane is based on polyalkylene diols end-capped with an isocyanate having isocyanate functionality greater than two. Such construction adhesives are typically applied by roll coater or bead applicator.
For other applications, thixotropy is a desirable characteristic, i.e., reduction in viscosity when subject to shear forces and a return to a more viscous state upon application. For example, thixotropy is desirable for an adhesive that is to be applied by caulk gun. Thixotropy is currently provided to such adhesives by the addition of waxes, fumed silica and other inorganic fillers. Such thixotropic agents may detract from the bond strengths of the construction adhesive. Also, such thixotropic agents are relatively expensive.
Another characteristic of such polyurethane adhesives is their tendency to foam, a characteristic which is disadvantageous in certain applications. For example, in producing a pre-fabricated stud framework, excessive foaming of the bondline will produce undesirable spacing between the framework pieces.